Use of snail neurons in developing quantitative ultrastructural parameters for neurotoxic side effects of Vinca antitumor agents.

The central nervous system of the snail Lymnaea stagnalis was studied in order to develop a test system to predict the neurotoxic side effects of the three cytostatic Vinca alkaloids, vincristine (VCR), vindesine (VDS), and vinblastine (VLB). Vinca alkaloids appear to interfere with microtubule formation by the induction of paracrystalline inclusions. After in vitro incubation the numbers of these inclusions were counted in cross-sections of the cerebral commissure using electron microscopy. For each compound the number of paracrystalline profiles increases with increasing concentrations and incubation times. At equimolar concentrations (0.15 mM), VCR induces more paracrystals than VDS, and VDS induces more than VLB. These effects are clear after short periods of incubation (e.g., after 2 h, VCR:VDS:VLB = 5:2:1). Equitoxic concentrations of VCR, VDS, and VLB induce similar numbers of paracrystals. Furthermore, morphological changes in the cell bodies of identified neurons (light green cells) in the cerebral ganglia were observed. Quantitative analysis shows that at equimolar concentrations the surface area of nuclear chromatin of all Vinca alkaloid-treated cells is approximately 30% lower than that of the controls. The lamellae of the rough endoplasmic reticulum are swollen and have lost their regular arrangement. For VDS and VLB this swelling is accompanied by a strong increase (about 3-fold) in the total surface area of the rough endoplasmic reticulum. No increase was observed for VCR. The compounds do not affect the number of secretory granules. In contrast to the controls, all Vinca-treated cells show lipid droplets. After VCR treatment they are about 5-fold as numerous as after treatment with VDS or VLB. The total surface area of lysosomes increases about 1.3-fold by VDS and VLB treatment and about 3-fold by VCR treatment. From these quantitative data it is concluded that VCR is more neurotoxic than VDS and VLB. VDS appears to be more neurotoxic than VLB as judged from the data on paracrystal induction. On the basis of a comparison of these data with clinical data on Vinca-induced neurotoxicities, it is proposed that neurons of the snail L. stagnalis may be suitable for the development of a test system to predict the degree of clinical neurotoxicity induced by Vinca antitumor drugs.